Conventionally, for measurement of a physical parameter, such as an acceleration or an angular velocity, the electronic circuit includes a capacitive sensor connected to an electronic interface capable of outputting at least one measurement signal. The capacitive sensor is formed of two differential mounted capacitors whose common electrode is movable and forms part of an armature elastically retained between the two fixed electrodes.
With this differential capacitive sensor, it is possible to make a measurement along one direction of motion of the moving electrode. At rest, the common moving electrode is equidistant from the two fixed electrodes in order to have two capacitors with an equal capacitance value. The common moving electrode can move some distance in the direction of one or other of the fixed electrodes under the action of a force. Thus the capacitance value of each capacitor varies inversely. The electronic circuit therefore generally makes it possible to output an analogue signal. This analogue output signal takes the form of a voltage dependent on the capacitance variation of the two capacitors.
The capacitive sensor may be a uniaxial accelerometer with two differential capacitors for making an acceleration measurement together with the electronic interface. However, it is also possible to provide three pairs of differential capacitors for such a capacitive sensor, such as a triaxial MEMS accelerometer for making a measurement in three directions X, Y and Z. A triaxial MEMS accelerometer of this type may include a single mass, i.e. an inertial mass common to the three pairs of differential capacitors, or three masses for the three pairs of capacitors.
The electronic interface and the capacitive sensor are generally each made in a semiconductor substrate. Thus, stray capacitances are added to the capacitances of the capacitors of the MEMS capacitive sensor, which creates non-linearities and also reduces the sensitivity or gain of the electronic circuit. The stray capacitances of the MEMS capacitive sensor together with those of the electronic interface result in a variation in the measured actual force, which is a drawback.
EP Patent Application No. 2 343 507 A1 discloses an electronic circuit with a uniaxial or triaxial measuring sensor. The measuring signals are digitally processed after a charge transfer amplifier in a logic unit. Following positive polarization and negative polarization of the capacitive sensor, digital measurement signals are stored in corresponding registers of the logic unit. A digital-to-analogue converter is also used to successively convert the digital signals for each axis in a measurement cycle into a voltage to be applied to the sensor electrodes via a switching unit. Combining the digital measurement signals after positive and negative polarization makes it possible to reduce the effect of the stray capacitances.
To obtain final acceleration measurement values for each axis, a dichotomy algorithm is used in the logic unit for a certain number of measurement cycles, prior to finishing with oversampling steps. With this dichotomy algorithm, the measurement always starts, during each conversion, at half the measurement voltage range. If an error occurs during the first measurement with a large change step in the logic unit, the final value at the end of all the measurement cycles will inevitably be erroneous, which constitutes a drawback.
Further, with an electronic circuit having a capacitive sensor, for each measurement cycle, an electrode polarization phase must always be performed, followed by a phase of discharging the capacitor electrodes by the converter measurement voltage. Consequently, with positive polarization and negative polarization, four phases must be provided per measurement cycle. This slows down the supply of digital measurement signals at the end of each conversion, which constitutes another drawback.
US Patent Application No. 2013/0088247 A1 may be mentioned, which discloses a measurement device with an array of variable resistance pressure sensors. The measurement device includes a switching circuit connected to the pressure sensors, a measurement circuit and a control unit for controlling the switching to selection of a target sensor of the array. Measurement is relatively complicated with this measurement device and does not concern an acceleration measurement.
U.S. Pat. No. 5,726,480 discloses the production of an accelerometer, which uses small masses connected to piezoresistors for the measurement. A silicon substrate is machined to form three-dimensional structures of the accelerometer with the piezoresistors on support arms. The piezoresistors are electrically connected in a Wheatstone bridge configuration for the acceleration measurement. However, no information is mentioned as to how to perform a measurement with the accelerometer.